The present invention relates generally to the field of telecommunications and, in particular, to a variable equalizer.
Cable networks are a common medium for providing video, audio and other data or programming to subscribers. Conventionally, cable networks have one or more head ends that transmit signals to subscribers over a medium that includes coaxial cable. Cable networks typically provide subscribers with programming on a variety of channels. Each channel occupies a distinct frequency band on the coaxial cable.
Signals transmitted over a coaxial cable tend to attenuate or decrease in signal strength or magnitude as the signals propagate away from the head end. Cable networks typically include amplifiers that are placed at various locations in the cable network. The amplifiers increase the magnitude of the signals to compensate for the attenuation due to the distance that the signals have propagated.
The attenuation in the magnitude of signals transmitted over coaxial cable further varies with the frequency of the signals. This is due to the xe2x80x9cfrequency responsexe2x80x9d of the coaxial cable. Significantly, the frequency response of a particular coaxial cable depends on the parameters in the construction of the coaxial cable such as, the characteristic impedance of the cable, the inner diameter of the outer conductor, the outer diameter of the inner conductor, the strand factor (K1), the braid factor (K2), the power factor, and the dielectric constant of the insulator of the cable among other parameters. Generally, however, the frequency response of a coaxial cable has a downward or negative slope with increasing attenuation as frequency increases as shown at 102 in FIG. 1. This is often referred to as xe2x80x9ccable tiltxe2x80x9d or xe2x80x9ccharacteristic attenuation,xe2x80x9d e.g., the cable loss at a maximum frequency for the cable network subtracted from the cable loss at the minimum frequency for the cable network.
To compensate for cable tilt, the amplifiers each have an equalizer circuit. The frequency response of the equalizer generally has an upward or positive slope with attenuation decreasing with increasing frequency, e.g., the reciprocal of the frequency response of the coaxial cable. A typical frequency response for an equalizer is shown at 100 by way of example in FIG. 1. The cascaded response of a coaxial cable and the equalizer is shown at 104 in FIG. 1. Thus, when properly adjusted, an equalizer compensates for the tilt of a coaxial cable such that the signals in the entire frequency spectrum of the network have a substantially constant attenuation. This is described, mathematically, in equation (1):
K=KC(f)+Keq(f)xe2x80x83xe2x80x83(1)
In equation (1), Kc(f) is the characteristic attenuation or tilt of the coaxial cable and Keq(f) is the characteristic attenuation of the equalizer. The cascaded attenuation due to these two factors is represented by the constant K. This means that the cascaded effect of the cable and the equalizer is independent of frequency over the range of frequencies transmitted by the cable network.
Conventionally, the equalizer circuit can have a fixed or variable frequency response. Fixed equalizers typically exhibit better frequency performance than variable equalizers. However, the fixed equalizers are less versatile and thus require service providers to maintain larger equipment inventories and can cause service disruptions when repairs/upgrades are made to the network.
The frequency response of a variable equalizer can be adjusted to compensate for the tilt in a limited variety of coaxial cables. Traditional variable equalizers use one control signal to adjust the frequency response of top and bottom branches of the equalizer. The equalizer essentially can be represented by a two port bridge xe2x80x9cTxe2x80x9d network that exhibits the principle of duality. When the control signal is adjusted, the top and bottom branches of the equalizer change in tandem. Thus, the top and bottom branches are not independently controlled. This is done to reduce the complexity of the analog control circuit. As a result, traditional variable equalizers face numerous frequency limitations. These limitations include high insertion loss, poor return loss, flatness (normalized to cable shape), and limited tilt settings (tunable tilt range).
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved variable equalizer.
The above mentioned problems with equalizers and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A variable equalizer is described which provides for independent control of a number of breakpoints in the frequency response of the equalizer so as to allow the equalizer to be readily tuned to compensate for the tilt of coaxial cables over a wide range of characteristic attenuation.
In one embodiment, the variable equalizer includes a two port bridge xe2x80x9cTxe2x80x9d network with variable top and bottom branches that are independently and selectively adjusted to create a desired frequency response. For example, in one embodiment, the top branches include a number of variable RC networks and the bottom branches include a number of variable LR networks. In some embodiments, PIN diodes provide the variable resistance in these LR and RC networks. PIN diodes include P-type Intrinsic and N-type regions and function as variable resistors.